1. Field of the Invention
The present invention relates to a manually opened and closed stop valve.
2. Description of the Related Art
The manually opened and closed stop valve of this type is adapted to bring a closure member into closing contact with a valve seat by a thrust of a thrust screw member, which results from advancing the thrust screw member to a housing through a tightening rotation of a handle.
However, when the handle is excessively tightened by mistake, the thrust screw member produces an overmuch thrust, which excessively pushes the closure member to the valve seat. This causes either or both of the valve seat and the closure member to make plastic deformation, which results in a likelihood of worsening the shut-off efficiency of the stop valve.
In order to prevent the foregoing excessive tightening, it is considered to prohibit the thrust screw member from further moving in a direction for valve closing over a predetermined distance, by a stopper fixed within the housing.
In this case, however, there are produced the following new problems.
For any reason, a foreign matter may bite into a space between the closure member and the valve seat. Or if the opening and closing operation is repeatedly conducted, it causes a sealing surface of the valve seat to wear off too much. In these cases, the closure member needs to be brought into closing contact with the valve seat more strongly.
However, as mentioned above, provided that the stopper prohibits the thrust screw member from further moving for valve closing over the predetermined distance, it is impossible to conduct further extra-tightening, which results in failing to satisfy the above-mentioned need.